The present invention relates to a die assembly and a method for extruding a wood-thermoplastic composite material with improved dimensional stability.
Composite materials consisting of a mixture of wood particles in the form of sawdust and a thermoplastic material have been known for many years. The materials so formed may be used in many of the same applications as an all wood product but offer the advantages of providing high resistance to rot, insects and moisture. These products can have the same workability as wood and are splinter-free. However, these materials do not exhibit the same physical characteristics as wood and therefore may not be used as structural members in some applications. The recent past has seen increased interest in composite material manufacture as a viable outlet for recycled post consumer thermoplastic materials. This interest has been spurred by the prospect of environmental regulations mandating the recycling of these materials. Also valuable, ever-shrinking landfill space may be conserved if both spent sawdust and plastic material are reused rather than disposed.
Forming a wood-polymer composite into a final product has been accomplished using some of the techniques used for forming all polymer products, including extrusion. While the technology for extruding all polymer products is well-developed with fairly predictable results, the extrusion of a wood-thermoplastic composite material using recycled input materials is subject to much wider variance in the molecular makeup and physical characteristics of the input materials, depending on available sources of recycled material. Moreover, a wood-thermoplastic composite has unique melt flow characteristics, which prevent the literal translation of polymer extrusion techniques for use in composite material extrusion. The present invention is concerned with a die assembly and an associated extrusion method for obtaining improved dimensional stability of an extruded wood-thermoplastic profile. A profile is defined in this art as an article fabricated from the composite material, which has a variable length and consistent cross section. As used herein, the term dimensional stability refers to the tendency of a finished profile of a wood-thermoplastic composite material to hold to predetermined cross-sectional dimensions along its entire length.
There are particular problems encountered with wood-thermoplastic profile extrusion that make control of the dimensional stability of the final product difficult. First, it is well known in thermoplastic extrusion that extrusion equipment and operating conditions must be closely matched to the material being extruded. For example, attempting to extrude a given thermoplastic such as polyethylene in equipment designed for a different type of thermoplastic such as polypropylene will give extremely unsatisfactory results. In contrast, the same extruder is used to make a composite product utilizing a wide range of thermoplastic input materials in the practice of the present invention. When fabricating a composite product using recycled thermoplastic input material, the nature of the thermoplastic material may change from one minute to the next requiring changes to process controls. The process can be unstable.
A second problem is the presence of volatiles in the wood component. Volatiles in the wood-thermoplastic mixture are heated in the extruder barrel and attempt to escape as the profile exits the die. This effect is referred to as foaming and can result in severe dimensional distortion (sagging, bending, bunching or stretching) of the profile. The kinds of volatiles that can be found in a wood-thermoplastic mixture typically include ordinary moisture and wood lignin, and terpenes. Another problem arises from the portion of the wood component that is near the surface of the profile exiting the extruder. At the profile""s high temperature, the wood component can oxidize as it is exposed to air, creating a burned appearance on the final product.
Underlying and to some extent magnifying these problems is the tendency of any extruded material to experience a phenomenon called die swell. Die swell is the tendency of an extrudate to swell as it leaves the die due to the release of shear stresses built up in the extrudate as it travels along the extruder barrel and die surfaces. Shear stress develops between the extrudate the die wall causing tears and roughness in the finished surface of the profile in addition to die swell. In ordinary polymer extrusion the shape of the opening at the die exit may have to be changed dramatically from the desired final shape in order to account for extrudate swelling.
Calibration processes have been developed for thermoplastics extrusion to ensure that the extrudate emerging from a die maintains a certain shape. Some of these techniques are described in Polymer Extrusion, C. Rauwendaal, (2d reprinted ed., 1990) at pp. 448-450. Calibration typically involves the use of some type of cooled sizing die to fix the shape of the extrudate. Calibration of thermoplastic extrudates typically requires exerting tension variable force on the extrudate to literally pull it through the calibration device. This approach is unworkable for the present invention because a wood-thermoplastic melt lacks sufficient melt strength to undergo appropriate tension. The extrudate of the present invention must be pushed through the die assembly and requires a different approach to ensure dimensional stability.
Various techniques are known for extruding wood-thermoplastic composite materials. U.S. Pat. No. 4,968,463 to Levasseur (the ""363 patent) discloses a process for producing extruded or molded objects from waste containing plastic materials. The process includes the steps of preliminary physical treatment, drying if the water content of the initial materials is over about 8% by weight, preheating to a temperature of 80xc2x0 C. to 160xc2x0 C. (176xc2x0 F. to 320xc2x0 F.), mixing or kneading at a temperature of 120xc2x0 C. to 250xc2x0 C. (248xc2x0 F. to 482xc2x0 F.) to form a paste, and injection molding or extrusion. The process also contemplates the addition of coloring and/or filling agents such as compost refuse, incineration plant cinders, old paper, ground rags or fines from household waste after the drying step. Levasseur also discloses the use of a water-cooled casing immediately after the die to cool the profile.
U.S. Pat. No. 5,217,655 to Schmidt discloses a process for preparing a composite product from an intake material incorporating commercial, municipal or industrial waste containing organic, plastic and fibrous material. The intake material is first granulated and sized and is then heated in three consecutive stages. The material is continuously mixed during the heating stages and reaches a final temperature of about 160 to 204xc2x0 C. (320 to 400xc2x0 F.). The heating and mixing steps are carried out under very high pressures to ensure encapsulation of the filler material by the plastic fraction. These high pressures are stated to range from about 141 to about 492 kg/cm2 (2,000-7,000 psi). The material may then be co-extruded with a reinforcement structure and formed into a final product using vacuum profile dies or compression molding. The patent discloses mist spray cooling tanks as a preferred method of cooling the final product.
U.S. Pat. No. 5,082,605 to Brooks et al. discloses a method for making a composite material comprising a discontinuous phase of cellulosic fiber encapsulated in a polymeric matrix. The method requires that cellulosic fiber and polymeric material be mixed while simultaneously being heated to an encapsulation point. The mixture consists of a discontinuous phase of cellulosic fibers encapsulated in a polymeric continuous phase. The mixture is then reduced mechanically into smaller golf ball sized particles having a maximum dimension of about 3.8 cm (1.5 in). The resulting particles are then extruded into a homogenous mass while maintaining the particle temperature within the encapsulation range. The homogenous mass is then forced through an alignment plate to substantially align fibers in a flow direction and then through an extrusion die to form a composite extrudate.
U.S. Pat. No. 4,225,640 to Erb discloses a method for reclaiming thermoplastic material from municipal waste. The thermoplastic material is first separated from the municipal waste and is decimated into fragments of a particular size. In similar fashion fallen leaves are separated from other municipal waste and are shredded to a predetermined size. The size to which both components are decimated is measured by the ability of the fragments to pass through a particular mesh opening. The fragments of thermoplastic material and leaf material are then mixed thoroughly and subject to sufficient heat and pressure so that the thermoplastic material coats and bonds to the surfaces of the leaf fragments. The unitary mass so formed is then permitted to set and cure so as to be formed into any desired shape. The patent discloses that a preferred shape is the form of particle boards of standard size. The patent also discloses that the unitary mass may be extruded to form finished articles. The flow of the mixture through the extruder is preferably controlled to provide a desired orientation of the leaf fragments as they flow therethrough.
Despite the presence of these prior art processes, it remains a challenge to consistently and accurately fix the dimensions of a continuously extruded profile composed of a wood-thermoplastic composite material.
The present invention overcomes the disadvantages of the prior art by providing a die assembly method for making a dimensionally stable wood-thermoplastic composite material of constant quality despite using input materials of varying quality and makeup. The invention relates to a die assembly for making a wood-thermoplastic composite article comprising a converging die means for forming an extrudate profile of a desired cross sectional shape, the die means having an entrance end and an exit end; a low friction insulating land section secured to the exit end of the converging section and adapted to receive the profile through a land section passage, wherein the land section passage has a cross sectional shape substantially similar to the cross sectional shape of the die means exit end and a low friction surface in contact with the extrudate; and an enclosed cooling section secured to the low friction land section comprising: i) a cooling medium inlet section having a passage adapted to receive the profile, ii) at least one cooling medium channel disposed within the inlet section, the cooling medium channel extending radially from the inlet section passage, and an enclosed cooling flume secured to the cooling medium inlet section and having a flume passage therethrough, wherein the flume passage is in communication with the cooling medium inlet section passage. The cooling medium inlet section may further include a plurality of cooling medium channels in communication with the cooling medium inlet section passage.
The present invention also relates to a method for making a dimensionally stable wood-thermoplastic composite material composed of a wood component and a thermoplastic component comprising the steps of: a) forming a wood-thermoplastic mass, the mass being at a temperature above the melting temperature of the thermoplastic component; b) passing the mass through a converging die to form a shaped extrudate profile having a cross sectional shape; wherein at least a part of the converging die is maintained at a substantially higher temperature than that of the mass, c) feeding the profile through a low friction thermally insulating land section, the land section having a cross sectional shape substantially similar to the cross sectional shape of the exit end of the converging die; and d) quenching the profile in a non-oxidizing fluid to form a substantially dimensionally stable outer shell around the profile, the shell having a temperature below the melting temperature of the thermoplastic component. The quenching step may be accomplished by contacting the profile with a cooling medium in a cooling means, the cooling means including: a) a cooling medium inlet section having a passage therethrough, the passage adapted to receive the profile, b) at least one cooling medium channel disposed within the inlet section, the cooling medium channel extending radially from the inlet section passage, c) an enclosed flume secured to the cooling medium inlet section and having a flume passage therethrough, the flume passage having an interior surface, wherein the flume passage is in communication with the cooling medium inlet section passage; and d) a cooling medium annulus defined between the flume passage interior surface and the profile.
It is therefore an object of the present invention to provide an economical process for making a dimensionally stable wood-thermoplastic composite material.
It is another object of the present invention to provide a die assembly having a low friction land section which separates a hot converging die section from a cooling section adapted to quench a hot wood-thermoplastic profile.
Still another object of the present invention is to provide a method for quenching a hot, wood-thermoplastic composite material profile in a non-oxidizing environment so as to prevent a burned appearance on the surface of the finished composite.
Still another object of the present invention is to provide an extrusion method that allows an extrusion die to operate at very high temperatures adjacent to a cooling section that quenches a profile at very low temperatures to produce a finished product having much improved surface quality.
Yet another object of the present invention is to provide a wood-thermoplastic composite method that is highly tolerant of momentary melt flow stoppages.
It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and, together with the description, serve to explain the principles of the invention.